a) Field of the Invention
The invention is directed to an arrangement for optimal current generation in processes of electrochemically initiated plasma-chemical layer production. Above all, it is applicable in the plasma-chemical conversion of electrochemically pre-formed layers, e.g. on light metals or their alloys.
b) Background Art
It is generally known that plasma-chemical processes can take place as parasitic processes in the formation of electrolyte capacitors with direct voltage. The patents DD 142 360, DE-OS 23 60 630 and DE-OS 23 60 688 describe the deliberate use of this effect in forming or reforming electrolyte capacitors. After a forming voltage passes through, which is material-dependent and often less than 100 V, the direct voltage is increased until the plasma discharge is initiated. A disadvantage in this solution consists in that the produced layers have considerable roughness and different layer thickness.
To eliminate such disadvantages it was suggested in U.S. Pat. No. 4,868,789 to carry out the process of electrochemically initiated plasma-chemical layer production with pulsed current.
The use of devices for producing current or voltage pulses is known for a number of electrochemical processes of varying complexity, e.g.
for electroplating--DE-OS 25 41 528; DE-OS 26 04 628 PA0 for anodizing/anodic oxidation--DE-OS 33 05 355 PA0 for electrolytic etching--DE-OS 15 64 486 PA0 for forming capacitor foils--DE-OS 14 89 695 PA0 for electrical discharge machining--U.S. Pat. No. 4,776,281 PA0 the dependency of the curve shape of the output voltage on the pulse magnitude selected in each instance; PA0 the rigid linking of the frequency of the output voltage with the mains frequency; PA0 the interdependence of the duty factor and pulse voltage; and PA0 the uneven, unfavorable mains load, although the elimination of interference in such current inverters on the line side is viewed as technically solved. PA0 the use of fixed frequencies already mentioned in the first variant of the construction of pulse systems; PA0 the very high, necessary symmetry requirements for the utilized electrodes (identical parts with identical preliminary treatment); and PA0 the poor reproducibility of the layers due to disproportionation effects.
A series of technical principles are known for the construction of pulse systems. One variant is line-commutated current inverters without an intermediate d.c. circuit and energy storage according to the principle of phase control, e.g. for controlling motors of all performance classes or for lighting and heating systems. The disadvantage of this solution for use in electrochemically initiated plasma-chemical layer production consists in
Another variant for the construction of pulse systems consists in master-controlled current inverters with an intermediate d.c. circuit. The disadvantages of the above variants can accordingly be overcome.
In order to produce pulses of different pulse voltage or current from a voltage fixed by the main rectification, it is necessary to use storage elements (generally inductances, sometimes also in combination with capacitors), e.g. blocking converters as are described in DE-OS 3 040 481. They are used to produce a d.c. voltage supply after rectification and smoothing of the pulses (switching network circuits).
Disadvantages result when applied in electrochemically initiated plasma-chemical layer production. These include the occurrence of very different load ratios. For this reason the energy coupled into the inductance must be made dependent on the load ratios. This necessarily leads to an operating frequency which is dependent on the load to a great extent, a different duty factor, and a coating technology which is not reproducible in an exact manner. Further, the coating parameters can be freely selected only to an extremely limited degree. Still further, the time curve of current and voltage cannot be compensated for during short-term load fluctuations such as are known to occur in this process. Even further, there is no assurance of a limiting of the voltage peaks occurring in the pulse and local defects in the layer are unavoidable for this reason. Finally, the voltage peaks occurring in operation lead to increased demands on the switch elements (high current load capability in connection with high voltage strength).
It is known from verbal communications of the Technical University of Chemnitz to dispense completely with a conversion in processes of electrochemically initiated plasma-chemical layer production and to apply alternating current or three-phase current directly to the electrode system. Disadvantages in this solution are particularly:
Accordingly, this method is generally unsuited for precision parts in precision device technology.
It should be noted that the electrical process implementation in electrochemically initiated plasma-chemical layer production is handled very differently and was inadequately incorporated into the evaluation of the produced layers. In addition to the characteristic of the utilized electrolyte systems, however, it is the second most substantial influencing variable on the process of electrochemically initiated plasma coating. It is generally known that two process steps exist which pass from one to the other through an increase in the electrode voltage. The first step is the so-called forming phase which is a purely electrochemical process and usually proceeds at voltages of less than 100 V. At higher voltages, plasma discharges occur accompanied by the formation of sparks which realize the actual layer formation. The particular disadvantage consists in the empirical character of the previously known strategies for conducting the process which impedes the determination of defined technological conditions.